1. Field of the Invention
The present invention relates to a method and apparatus for comparing images of the same object.
2. Description of the Related Art
The background will be discussed in the context of one particular application: mammography.
Mammography has been film-based for many years and there are now many different manufacturers of X-ray equipment for mammography. This in itself makes comparison of images of the same breast taken on different X-ray machines difficult, as there is a lot of variability in the imaging equipment, film and more simply, acquisition parameters. Over the last 2 years however the introduction of digital mammography in several forms has started to generate digital images instead of films and radiologists are now faced with the task of comparing films and digital images. This is clearly not easy to do and many quotations in academic and clinical papers have now revealed how much radiologists dislike doing this.
The situation might arise in many different situations, e.g.:                comparing films from X years previously with films from today;        comparing films from X years previously with digital images from today;        comparing films from a screening centre with digital images taken in an assessment clinic; and        comparing digital images from one system to digital images from another system.        
Currently, films and digital images are compared by putting the film on a light box next to the computer screen on which the digital image is displayed. The display is obviously completely different, one being back-projection with a particular luminance and the other being a fluorescent display with a specific gamma. The apparent brightness and contrast of the mammogram would then be different from one display to the next. Morover, some digital equipment performs some post-processing of the data. All this makes the objective comparison of, for instance, breast density, virtually impossible.
On top of that, it is difficult to compare images using a digital mammography workstation because the parameters may have to be re-set whenever other digital mammograms are imported. A further issue is that there are technically significant differences between the ways images are acquired from one digital system to the next. The image produced by mammographic equipment depends on:                1) the X-ray emitter characteristics (voltage, current, anode metals, etc.)        2) the X-ray absorption characteristics of the breast,        3) the transfer of X-Ray energy to detectable photons;        4) the transfer of photon into electrical impulse on the CCD (Charge coupled device), if the equipment uses any.        
There are typically half a dozen different technologies available in digital mammographic equipment (Cesium Iodide with amorphous silicon, phosphor, selenium, etc), all of which produce different images because even if step 2) is supposed to remain constant over time, steps 1), 3) and 4) relate to physical interactions of materials, which are different from equipment to equipment.